onsdag 21 juli 2010

Can You See the Earth on the Surface of the Sun?

We all know that the Sun is clearly visible on the surface of the Earth, because the Sun is big and very hot.

In a discussion about backradiation, one may ask if the Earth is visible on the surface of the Sun, recalling that the Earth is small and lukewarm.

A rough calculation shows that the Earth blackbody radiation at surface of the Sun has to compete with a radiation intensity which is almost 10 orders of magnitude bigger.

A sensor resolving 10 orders of magnitude requires a resolution to atomic scales, which seems beyond what is thinkable. Just a reflection stimulated by a comment to the post Clouds as Retailers of Heat Energy.

We learn that it is easier to see a hot blackbody from a cold blackbody, than a cold from a hot.

This is because the high frequency signature of a hot blackbody is visible against a cold low frequency background, while the low frequencies of a cold blackbody disappear into a hot background.

18 kommentarer:

1. the radiation from Earth isn't "diluted", it just covers a smaller and smaller patch of sky as you get further away. Just make a telescope the focus on that patch and you can see it from very far away.2. The radiation of the Earth and from the sun is coming from opposite directions, thus it is no great difficulty seeing the Earth from the surface of the Sun, all you need is some directional sensitivity, and a cooled detector. Of course, the Sun doesn't really have a surface, so there is going to be a hot atmosphere above you obscuring it, but if it had a fixed surface this would work.

You might as well ask if it is possible to see distant stars from Earth considering how diluted their light is, and I think you know the answer to that one.

You don't see planets mainly because the are so close to their star as seen from Earth. Then the distances involved are a tad bit longer too. As usual your examples and analogies fall flat on their face at closer examination.

Claes, no you don't have a point as I explained. It's only illuminating to see analogies in situations that are actually analogous. Your analogies are just weird. This one was at least within science, the one with the Swedish tax system was deranged.

Claes, 9 orders of magnitude is less than 2**32. It is not hard to achieve 32 bits of dynamic range. I doubt it would be hard to detect the Earth from the Sun. And planets have been directly imaged around other stars.

The relevance of this is not clear. Your quixotic campaign against back radiation is absurd, and your bizarre analogies are irrelevant. Trivial observations prove you wrong. To persist with views that counteract observations and all common sense is idiotic.

Roger, I don't think anyone denies "backradiation" per se. Of course there will be radiation going in all directions. The problem I have with your kind of backradiation is that it is not treated as a heat transfer which operates under the same rules as heat conduction. If you place a hot object besides a cold object heat conduction will make the to objects have the same temperature after a while. Yet your backradiation seems to operate the other way around. A stratosphere initially held at the same temperature as the surface will cool while the temperature at the surface increases. You say that only common sense is required to understand this, yet apparently it took climatologists some 100 years to work it out.

Anders, you are ignoring the very subject of this thread: the sun. It's the energy input from the sun that creates a temperature gradient in the atmosphere, without it temperature would equalize after a while.

Since the sun would exist even without greenhouse gases, should I interpret your answers as that there would be a temperature gradient without the greenhouse effect? Moreover, why is the radiation from the sun "energy" while the outgoing radiation from the earth is "heat"?

Of course there can be a temp grad without GHG. By gravitation, in principle without energy input: A solution to the Euler equations with zero forcing. Well, heat is a form of energy and so is radiation, so maybe I mix them up.

Anders, it's impossible to guess from what level of ignorance you are asking these questions. To help me out, please consider three cases. First, a planet with no atmosphere. Second, a planet with an atmosphere that is transparent to all EM radiation. Third, a planet with an atmosphere that is transparent to optical but opaque to infrared. Tell me qualitatively how the surface temperature would differ between the cases.

Third case: A temperature gradient starting at 15 degrees C going down at a rate of approx 6.5 degrees C. If there was no convection the surface temperature would be about 80 degrees C and a lapse rate of approximately 16 degrees.

Did I pass the exam?

Now, it is peculiar that this strange thermodynamic phenomenon seems to appear only at large distances in a gravity field. Moreover it seems to appear on all atmosphere bearing planets, including Jupiter.

As for my question. If the heat input from the sun is indeed an "energy" source, then there would be no violation of the 2nd law to suppose that the energy is then transported by a temperature gradient to the effective radiative surface of the atmosphere. Thomas used precisely the same logic with the sun, remember? If you now say that oxygen and nitrogen cannot radiate, then my only comment would be: neither can hydrogen nor helium.

To elaborate a little bit more on the point I wanted to make. It is important that we all make clear in our minds the difference between an equilibrium system and a system out of equilibrium, also to distinguish between organized energy and heat. If the heat from the sun is treated as an energy source like if the heat was created from an energy source in the interior of the earth then what we deal with is a problem of heat conduction by a temperature gradient. In the first case, if we assume that a non GE atmosphere would be isothermal it would imply that it had infinite heat conductivity. If on the other hand the heat from the sun is treated as, yes, heat, then I see no logic in not assuming that the atmosphere would be isothermal with or without greenhouse gases.

Thomas, I don't doubt that you have more knowledge about absorption spectra than me. Do you know claim that there is indeed a greenhouse effect in the sun?

Anders, a non GE atmosphere would be isothermal because it can't lose any heat to space! It will get exactly the same temperature as the surface because that's the only place it can pick up or get rid of heat. No "infinite heat conductivity" is required.

"Do you know claim that there is indeed a greenhouse effect in the sun? "

Not as conventionally defined although obviously physics is the same everywhere. The greehouse effect is based on the fact that our atmosphere is transparent to visible light from the sun but absorbs IR emitted from the Earth. In the sun, however, the heat is produced in the center. All that is needed to keep the center hot is that the body of the sun does not allow radiation to escape directly.

Anders, it seems on the face of it that you do understand what the greenhouse effect is, and how it works. However, when you say "it is peculiar that this strange thermodynamic phenomenon seems to appear only at large distances in a gravity field" I don't understand what you mean. Gravity is irrelevant. Also your point about the Sun being an energy source is not clear. Of course the Sun is an energy source.